1. Field of the Invention
Embodiments of the present disclosure are related to systems and methods for calibrating power, and in particular to a system and method for automatically calibrating output power of a communication device.
2. Description of Related Art
In a wireless communication system, the distance between a communication device and a radio base station is not constantly fixed. Therefore, automatic adjustment of an output power of the communication device is necessary to maintain clear reception coverage of the radio base station and sufficient capacity of a wireless communication system according to the distance between the communication device and the radio base station.
At present, there are several major frequency bands in use by Global system for mobile communication (GSM) communication devices, such as 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz bands. In each of the major frequency bands, there are 15 or 16 transmitting power classes. Furthermore, each of the transmitting power classes has corresponding auto-power calibrate (APC) values, which are not the same. Generally, every communication device has its own APC List stored in a Flash read-only-memory (Flash-ROM) to record the APC values of different transmitting power classes and corresponding actual output power values. For example, FIG. 1 shows exemplary APC values for a frequency band of GSM 900 MHz. The list records the APC values and corresponding actual output power values, measured in Decibels per milliwatt (dBm), from a power amplifier of the communication device for each of the transmitting power classes on the 900 MHz frequency band.
To accurately change the output power of the communication device at a given time, the communication device, having selected a target output power value, first searches the relevant APC list to locate native APC values corresponding to the target value, converting the values to a voltage value, and directs the power amplifier to change the output power value accordingly.
However, due to individual physical characteristics of communication devices as manufactured, actual output power values differ from the theoretical output power values corresponding to the APC list. The difference may impact capability of the power amplifier of the communication device to accurately change the output power values. Usually, in order to solve this problem, some calibration processes are applied in production line to output power values corresponding to the APC list. One example of calibration is implemented in a fashion of recursive calculations along with a logarithmic formula “Power=a+b+c*log(APC)+d/(APC2),” where “Power” is the actual output power value of a power amplifier of a communication device, “APC” is one of the auto-power calibrate values in the APC list, and “a,” “b,”, “c”, and “d” are related parameters. However, due to the non-linearity of the logarithmic formula, the recursive calculations with a logarithmic formula usually is time-consuming, and sometimes ends up in a dead loop, resulting in an unreliable solution and unproductive performance of calibration process.
Therefore, what is needed is a system and method for remedying the above-mentioned limitations.